Microwave tube incorporating a coaxial coupler having water cooling and thermal stress relief



Ju y 9, 1968 u. R. WOLFF ETAL. 3,392,303

MICROWAVE TUBE INCORPORATTNG A COAXlAL COUPLER HAVING WATER COOLING AND THERMAL STRESS RELIEF Filed Aug. 4, 1964 n I 9 Q l f I 8* I av IQ 1L 3 18 m m "g N 1 N g :0 q 3 c I, N v g N c": c I f c 0 I 2 I a I m A a NZ A 1 I1 1 v I I I n: In I u i a: 2- I O O r o n N 2 I ID n I g 'Q N N\= Q N I I 2 2 9 n E a 5]- In INVENTORS N ROBERT L.WOODS ULRICH R.WOLFF BY%VZ% ATTORNEY United States Patent 3,392,303 MICROWAVE TUBE INCORPORATING A C0- AXIAL COUPLER HAVING WATER COOL- INC AND THERMAL STRESS RELIEF Ulrich R. Wolff, San Francisco, and Robert L. Woods, Palo Alto, Calif assignors to Varian Associates, Palo Alto, Calif, a corporation of California Filed Aug. 4, 1964, Ser. No. 387,454 16 Claims. (Cl. 315-539) ABSTRACT OF THE DISCLOSURE A Klystron tube having a coaxial output coupling device. Means are provided for water cooling a vacuum window in the coupler and for permitting thermal expansion without fracturing the window. The major axis of the coupler is oriented parallel to the beam axis along the outer envelope of the tube. An inductive coupling loop in the output cavity of the tube is connected to the center conductor of the coaxial coupler via conductive coupler blocks at a 90 angle to the center conductor. A shorting plane located on an extension of the center conductor at a distance of Mink forms a matching stub permitting manipulation of the Q of the output cavity. An enlarged portion of the center conductor in the vicinity of the window improves impedance transformation characteristics.

This invention relates in general to high frequency electron discharge devices and more particularly to high power transmitter tubes and electromagnetic wave coupling means usable therewith.

Divergent design requirements encountered in constructing such high frequency electron discharge devices as klystrons constantly demand the exercise of the inventive faculty in order to find a solution compatible with the often times conflicting design requirements. An example of such conflicting design requirements which was instrumental in the germination of the present invention is the following. Considering the requirements of minimizing the overall size and weight and thus cost of a solenoidal focusing structure for medium and high power transmitter tubes, such as for example, a multi-cavity klystron amplilier having coaxial R.F. coupling means, while simultaneously attempting to optimize the RP. coupling means for such devices with regard to obtaining a given design output cavity Q as well as good R.F. power transformation through the coupling means together with the requirements of handling medium and high powers while simultaneously preventing window failure in a coaxial coupler embodiment and it is apparent that a formidable array of complex physical and electrical problems must be dealt with. When the above design requirements are coupled with additional requirements such as stabilization of tube operating characteristics under diverse operating conditions as well as building in sufiicient strength and ruggedness to assure prevention of vacuum leakage in the RF. coupler portion of the tube, the design requirements are compounded.

The present invention succeeds in providing a solution to the aforementioned design requirements in a novel manner through the utilization of a coaxial coupler design which enables the tube designer to expedite the rather cumbersome chore of obtaining the proper Q for the output cavity to which the coaxial coupler is connected while simultaneously providing good power transfer through the coaxial coupler through the inclusion of stress relieving means and cooling means for the window portion of the coupler design. Specifically, the present invention allows reduction in solenoidal size and weight by utilizing a reduced diameter coaxial coupler section in the vicinity of the pole piece and output cavity and permits handling of medium and large RF. power levels by positioning the wave permeable vacuum window in an enlarged coaxial section which is coupled to the reduced or smaller diameter section. By utilizing an approximately Mth shorting plane in conjunction with a hollowed out center conductor having bi-directional fluid flow means incorporated therein in the reduced diameter section of the coaxial coupler, the present invention provides cooling and stress reduction means for the wave permeable vacuum sealed window portion of the coupler. Additional stress relief means are provided in the vicinity of the vacuum window itself to further enhance prevention of window failure due to such operational hazards as thermal stress rupture and dielectric breakdown due to the RF. power levels being coupled through the window. The utilization of an approximately AA shorting plane also permits ease of manipulation of the magnitude of the cavity Q parameter without appreciable variation in slope or shape of the Q characteristic over the operating band of the tube. By utilizing a sliding joint between the shorting diaphragm and the surrounding coaxial outer conductor the magnitude of Q can be easily adjusted to the chosen design value and then the sliding joint is vacuum sealed by any known metal sealing techniques. The impedance transformation characteristics of the coaxial coupler are further enhanced by the inclusion of a stepped center conductor in the vicinity of the wave permeable vacuum window.

Therefore, it is an object of the present invention to provide an improved high frequency electron discharge device.

A feature of the present invention is the provision of an improved high frequency electron discharge device and RP. coaxial coupling means therefore.

Another feature of the present invention is the provision of an electron discharge device including a coaxial coupler having a %Hhi"15% shorting plane together with fluid cooling means and stress relief means for a wave permeable vacuum window disposed in said coaxial coupler.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in conjunction with the accompanying drawings wherein:

FIG. 1 is an elevational view, partly cut away, depicting a multi-cavity klystron embodying the features of the present invention;

FIG. 2 is a fragmentary sectional view of the coaxial coupler portion of the klystron depicted in FIG. 1 encompassed by lines 22; and

FIG. 3 depicts an illustrative schematic view of a cavity resonator and coaxial coupler having a shorting plane together with a graphical portrayal of Q vs. frequency for a few illustrative shorting plane positions.

Referring now to FIG. 1, there is depicted a tunable multi-cavity broadband klystron amplifier 5 having a beam forming and projecting means (electron gun means) 5 disposed at the upstream end portion of the klystron. Disposed intermediate the electron gun means 6 and an electron beam collector means 7 at the downstream end of said klystron is a beam interaction region 8 including a plurality of tunable resonant cavities. Each of said cavities is provided with suitable tuning means 9 for controlling the resonant frequency of each cavity in accordance with known broadbanding techniques. Each of said tuners is provided with suitable tuner actuating means 10 which preferably take the form depicted in US. Patent 3,300,679 by Jack A. Brown issued January 24, 1967 and assigned to the same assignee as the present invention. A motor 11 is coupled to a drive shaft 12 by suitable gear- 3 ing arrangements 13 and to each tuner actuator by gears 14 such as, for example, described in the aforementioned patent, and serves to control tuner positions in a prescribed manner.

In order to obtain proper focusing of the electron beam a solenoid 64 surrounds the beam interaction region 8 and, by virtue of being mounted on magnetic pole piece 15 at the downstream end of the klystron and an equivalent pole piece at the upstream end, both of which are preferably of soft iron, provides a suitable magnetic field along the beam axis in a known manner.

A rather large R.F. coaxial coupler 16 is coupled to the output cavity 19 and serves to extract the amplified R.F. energy from the klystron in a manner to be described in more detail hereinafter. An input coaxial coupler 17 provides the input cavity with the R.F. energy which is to be amplified. The klystron illustrated in FIG. 1 finds usage in such applications as surface radar wherein high peak power R.F. pulsed energy is required over appreciable bandwidths. Careful attention must be paid by the tube designer in order to minimize overall tube weight and optimize efficiency of operation. This, of course, entails careful attention to the design of the RF. extraction part of the tube regardless of what type of tube is in volved. An attempt to provide means for eificiently extracting R.F. peak pulsed energy 300 kw. at duty cycles of S 4% in a coaxial coupler embodiment while simultaneously simplifying design of Q for the output cavity of a klystron resulted in the present invention. Ordinarily tedius cut-and-try processes are utilized to obtain the specified design value of Q for the output cavity of a klystron which value in conventional klystron design practice is 1.5R sn/o where R dynamic beam impedance as defined by QEXT bb b shorting plane which although not movable, even though flexible as will be explained hereinafter, in the final product results in enhancement of the final product by forming an integral portion of the coaxial coupler design as well as resulting in overall cost reduction.

Turning to FIG. 2, an enlarged sectional view of the coaxial coupler portion 16 of the present invention is depicted in detail together with a portion of a typical output cavity 19 and collector structure 7. The pole piece 15 has an off-center aperture through which the outer conductor 20 of the coaxial coupler 16 extends and is rigidly attached to by any suitable metal joining technique. The shorted AA stub portion of the coaxial conductor 16 is terminated by a flexible shorting diaphragm 21 preferably of stainless steel which is vacuum sealed, as by brazing to the hollow inner conductor 22 on the interior peripheral surface thereof and similarly sealed on the exterior peripheral surface thereof to an annular sleeve 23 which in turn is vacuum sealed as by brazing, to the interior surface of the outer conductor 20. A tubular manifold 24 terminates the end portion of inner conductor 22 which protrudes through the flexible diaphragm 21 exterior tothe vacuum side portion of the coaxial coupler 16. The manifold 24 provides a means of coupling cooling fluid from the hollow inner conductor 22 while simultaneously rigidly supporting fluid inlet tubulation 25 within the hollow inner conductor 22 as well as providing a fluid t'ight seal thereabout. The central axis Z of the coaxial coupler 16 is parallel to and spaced from the central axis Z of the tube.

Inlet tubulation 25 extends within and coaxially along substantially the entire length of inner conductor22 and terminatesshortof a sealing end plug 26. in an orifice portion 27, such as to providebi-directional flow of coolant within inner conductor 22. Sealing end plug 26 is fixedly secured to a rath'er'large annular metalblock 28 preferably of copper, which serves the multiple functions of simultaneously acting as a heat sink for an electromagnetic wave permeable'vacuu'm window 29 and as a matching network to.cornpensate for the impedance mismatch introduced by the ceramic window. Furthermore, metal block 28 serves to provide a seat for expansion cup member 30 which is preferably of Kovar and which serves to provide stress relief for the window 29. As seen in FIG. 2, the block 28 forms a cone on the window facing side thereof, which serves to provide a good thermal path for heat generated in the window to travel to the main body portion of the block 28 while simultaneously allowing the expansion cup member sufl'icient freedom to function as a stress relief member as well as an R.F. conductor. Window 29 is provided with a central aperture through which a preferably copper tubulation 31 extends for maintenance of electrical continuity. The apex 32 of the cone portion of block 28 terminates at the central axis of the tubulation 31 in a surface area 33 which is preferably smaller than the central aperture of the tubulation. All mating surfaces of the aforementioned elements are of course vacuum sealed by any suitable technique, such as brazing.

The apex surface 33 bears on the center expansion cup 30 and thus upon the occurrence of differential thermal expansion during operation of window 29 and the remaining portions of the coaxial coupler, adequate flexibility is provided to revent window fracture by a combination of good heat transfer from the window through block 28 and subsequent dissipation by means of cooling fluid flowing within the center (inner) conductor 22 and the incorporation of an expansion cup 30 as a part of the RF. feedthrough structure portion of the inner conductor in the vicinity of the window. A, practically speaking, identical expansion cup and block base member 34, 35 respectively, are similarly mounted on the air side of the window 29 with minor variations and functions as explained hereinafter, The expansion cup 34 has a central aperture through which tubulation 31 protrudes and the block 35 has a slightly extended apex portion which forms an interfitting rod 36 which is brazed or the like to the interior periphery of the tubulation 31. The portion of block 35 opposite the apex forms a cup within which a spring loaded combination male-female plug receptacle 39 sits which is securely yet slidably maintained therein by means of a helical spring 38 formed into a ring coaxial with the axis of the transmission line. A conventional mating male plug 41 completes the air side portion of the coaxial coupler 15 in conjunction with conventional slotted sleeve 42 and flange portion 43, which are secured to the enlarged outer conductor portion 44 of the coupler at the mounting flange portion 45 thereof. The window '29 exterior peripheral edge is vacuum brazed to a preferably Kov-ar sleeve 46 which in turn is brazed, or the like, to adjacent preferably stainless steel cup and sleeve portions 47, 44 respectively of the outer enlarged conductor portion 16" of the coaxial coupler 16. The Kovar sleeve 46 and expansion cups 30, 34 are utilized because of the rather close thermal coeflicients of expansion of Kovar and such wave permeable ceramics as alumina (AL 300), beryllia, single crystal sapphire, etc., which are advantageouslyutilized for the wave permeable window. Any suitable type of bracket such as 50 may be used to rigidly secure the coaxial coupler 16to the collector 7.

. OFHC (oxygen free high conductivity copper) is preferably utilized for the inner conductor portions, excluding the Kovar cups, of the coaxial coupler 16, while copper plated stainless steel is preferably utilized for the outer conductor portions of the coupler with the exception of the Kovar sleeve 45.

The above design utilizing an alumina window having a thickness of 0.200 inch mounted in a coaxial conductor having a nominal outer diameter of 3 /8 inches which was coupled to 1% inches nominal outer diameter coax at the cavity end with the coupler shorting plane or diaphragm spaced AA from the coupling axis C-C, wherein A is determined at the center frequency f of the operating band of the tube which in this particular illustration was 900 me. easily handled peak powers of 300 kw. at 4% duty cycles, having a pulse duration of 200 microseconds utilizing water (H O) cooling fluid pumped at a rate of around 1 gallon per minute at a pressure drop of 5 pounds per square inch.

Returning to the output cavity 19 portion of FIG. 2, a standard coupling loop 53 of cop-per is suitably mounted within the cavity 19 at the one end wall 54 and is coupled to the coaxial coupler along a coupling axis C-C through means of keyed coupler blocks 55, 56 which extend through coupling hole 57 in the cavity wall. A sleeve 58, in conjunction with coupler blocks 55, 56 forms a vacuum sealed coaxial junction between the cavity and the coaxial coupler 16. The particular shapes and sizes of coupler blocks 55, 56 in conjunction with the shape and size of loop 53 are manipulated on an empirical basis in order to predetermine the shape and slope of Q vs. frequency characteristic as depicted in the graphical portion of FIG. 3. Since these empirical techniques are known in the art and do not form part of the present invention, no further discussion thereof is warranted.

The schematic portion of FIG. 3, when taken in conjunction with the graphical portion of FIG. 3 illustrates the ease of manipulation of the magnitude of the Q vs. frequency characteristics while retaining the same slope and shape by merely varying the position of the shorting plane relative to the coupling axis (central axis CC of the coupler blocks 55, 56). A variation of 0.45 inch in D resulted in a measured change in Q magnitude of 12 at f where the D=1 characteristic is any arbitrary shorting plane lying within the flrn 10% range specified below although preferably at MA as determined at i The advantages flowing from the utilization of a AA stub in Q manipulation are hence obvious and a further discussion is not deemed necessary.

The distance between the shorting plane and the coupling axis C-C, according to the teachings of the present invention, is maintained within the following limits: /4n \il5%, where n is any positive odd integer and A is free space wavelength as determined at the center frequency f of the operating band of the electron discharge device. The exact configuration and geometric properties of the heat sink block 28 and expansion member or cup 30, with regard to providing proper impedance transformation characteristic for the transition between the reduced diameter first section 16' and the enlarged diameter second section 16", are determined by cut and try methods utilizing conventional impedance matching techniques after the first section 16 and coupling plane have been designed for the derived Q magnitude and coupling properties. Sufiice it to say that obviously many geometries could be utilized in the regions of the heat sinks 28, 35 and expansion cups 30, 34 to provide suitable impedance characteristics. However, the broad combination of impedance transformation with stress-relief both physical and electrical is taught by the present invention in a novel manner.

The utilization of a flexible diaphragm 21 for the shorting plane allows increased differential expansion between the window 29 and the remaining portions of the coaxial coupler without window failure or loss of vacuum through leakage at the various joints, which in the absence of said flexible diaphragm, would be subjected to thermal expansion stresses in far greater amounts which would obviously require either considerable increase in size and thus strength of the various parts and joints and/or complex and costly design of parts by selection of proper coeflicient of expansion materials, sizes, etc. The novel fluid cooling technique provides an easy method of maintaining window temperature within tolerable limits and preventing window failure. The placement of the window in enlarged size coaxial conductor provides increased power handling capabilities, while minimizing solenoidal weight, size and cost, by virtue of using smaller coax in the vicinity of the cavity and in the evacuated portions. The overall coupler design obviously has many varied advantages and quite obviously permutations and combinations of the individual features of the coaxial coupler taken alone and in conjunction with electron discharge devices of the klystron and traveling wave types are possible and are included within the scope of the present invention as set forth more broadly in the following clause.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A high frequency klystron device having electron beam forming and projecting means disposed at the upstream end of said device and a beam collector disposed at the downstream end of said device and a plurality of resonant cavities disposed therebetween in vacuum sealed relationship therewith; a coxial coupler including an inner conductor and and outer conductor, a shorting plane electrically connecting said inner and outer conductors at said high frequency and a wave permeable dielectric window vacuum sealed to said inner and outer conductors; said coaxial coupler being coupled to at least one of said resonant cavities at a coupling axis intersecting said central axis between said window and said shorting plane at a distance of A1I2A:15% from said shorting plane wherein A is free space wavelength as determined at the center frequency f of the operating band of said klystron device and n is any odd positive integer.

2. The klystron device as defined in claim 1 including means for fluid cooling said window.

3. The klystron device as defined in claim 1 including stress relief means integrated with the coaxial coupler in such a manner that said window is provided with differential thermal expansion stress relief whereby said window will not fracture in use.

4. The klystron device as defined in claim 1 wherein said shorting plane is a flexible diaphragm.

5. The klystron device as defined in claim 1 wherein said inner conductor of said coaxial coupler is hollow along a portion of the central axis of said coupler to conduct cooling fluid therethrough, said hollow inner conductor being coupled to a heat sink at the one end thereof, said heat sink being coupled to said window whereby heat from said window is transferred by conduction through said heat sink to said cooling fluid within said hollow inner conductor portion.

6. The klystron device as defined in claim 1 wherein said window has an aperture through the central axis thereof and wherein said inner conductor of said coaxial coupler extends through said aperture, said inner conductor of said coaxial coupler including at least one expansion member vacuum sealed to said window, said expansion member being adapted and arranged to provide stress relief means for said window whereby said window Will not fracture in use, said expansion member further serving as a current conductor for electromagnetic energy.

7. The klystron device as defined in claim 1 wherein said coaxial coupler includes a first section and a second section having an outer diameter which is larger than the first section, said vacuum window being disposed in said second section and said shorting plane terminating said first section at the end portion which is remote from said enlarged second section.

8. The klystron device as defined in claim 7 wherein said shorting plane is a flexible diaphragm and wherein the inner conductor of said coaxial conductor extending through said window and in the vicinity of said window includes a stress relief portion and a heat sink portion, said stress relief portion and said heat sink portion having a physical geometry adapted and arranged to provide impedance transformation between said first section and said second section.

9. The klystron device as defined in claim 1 wherein said coaxial coupler includes a first coaxial section coupled to at least one of said resonant cavities and a second coaxial section coupled to said first coaxial section, said second coaxial section having a larger outer diameter than said first coaxial section, the one end of said first coaxial section which is remote from said second coaxial section being terminated by said shorting plane, said shorting plane being a flexible diaphragm vacuum sealed across the end of said first section, and an inner conductor extending through the flexible diaphragm and the vacuum window along the length of said coaxial coupler and centered on the longitudinal axis of said coaxial coupler. said inner conductor having a heat sink portion and an expansion member coupled to said window on opposite sides of said window, said inner conductor having a hollow portion provided with means for passing cooling fluid bi-directionally through said flexible diaphragm along said inner conductor to one of said heat sinks whereby heat is transferred from said window to cooling fluid flowing in said hollow portion of said inner conductor.

10. The device as defined in claim 9 wherein said klystron is provided with solenoidal focusing means, said fousing means being coupled to said klystron by at least one magnetic pole piece, said magnetic pole piece being disposed downstream from said coupling axis, said magnetic pole piece having an aperture therein, said first section of said coaxial coupler extending through said aperture and said second section of said coaxial coupler disposed downstream from said pole piece.

11. A high frequency electron discharge device having beam forming and projecting means disposed at the upstream end of said device, beam collector means disposed at the downstream end of said device and electromganetic interaction means disposed therebetween, said means forming a vacuum envelope in conjunction with a coaxial coupler coupled to said interaction means, said coaxial having an electromagnetic wave permeable vacuum window disposed in vacuum sealed relation therein, said coaxial coupler having a flexible shorting diaphragm terminating one end thereof and means for fluid cooling said vacuum window incorporated in said coaxial coupler, 6

said cooling means including a hollow center conductor having bi-directional fluid flow provisions for coupling cooling fluid through said flexible shorting diaphragm along said hollow center conductor to the vicinity of said window.

12. The device as defined in claim 11 including an expansion member coupled to said window, said expansion member forming a portion of the inner conductor of said coaxial coupler whereby said window is further protected from fracture due to differential thermal expansion occurring in use.

13. The device as defined in claim 11 wherein said device is a klystron and wherein said interaction means includes a resonant cavity to which said coaxial coupler is coupled along a coupling axis therebetween and wherein said klystron is provided with solenoidal focusing means, said focusing means being coupled to said klystron by at least one magnetic pole piece, said magnetic pole piece being disposed downstream from the coupling axis between said interaction means and said coaxialcoupler, said magnetic pole piece having an aperture therein, said coaxial coupler having a reduced diameter first section coupled to an enlarged diameter second section, said first section extending through said aperture and said second section disposed downstream from said pole piece.

14. The device defined in claim 11 wherein said window has an aperture through the central axis thereof and wherein the inner conductor portion of said coaxial coupler extends through said aperture, said inner conductor portion of said coaxial coupler including at least one expansion member vacuum sealed to said window, said expansion member being adapted and arranged to provide stress relief means for said window whereby said window will not fracture in use, said expansion member further serving as a current conductor for electromagnetic energy.

15. The device as defined in claim 11 wherein said coaxial coupler includes a first section coupled to said electron discharge device and a second section having an outer diameter which is larger than the first section, said vacuum window being disposed in said second section and said flexible diaphragm terminating said first section at the end portion which is remote from said enlarged second section.

16. The device as defined in claim 7 wherein the inner conductor of said coaxial conductor extends through said window and in the vicinity of said window includes a stress relief portion and a heat sink portion, said stress relief portion and said heat sink portion having physical geometries adapted and arranged to provide impedance transformation between said first section and said second section.

References Cited UNITED STATES PATENTS 2,831,047 4/1958 Wadey 33398 2,994,009 7/1961 Schmidt et al. 313-24 X 3,054,925 9/1962 Walker et al BIS-5.48

FOREIGN PATENTS 126,844 2/ 1948 Australia.

0 ELI LIEBERMAN, Primary Examiner.

HERMAN KARL SAALBACH, Examiner.

S. CHATMON, JR., Assistant Examiner. 

